This invention relates to current control circuits for controlling the current supplied to an electromagnetic type actuator utilizing an electromagnet as an actuator element.
Generally, the operation of electromagnetic type actuators utilizing an electromagnet as an actuator element is controlled by the current supplied thereto. Thus, it is necessary to control the level of current precisely in order to ensure the desired operations of electromagnetic type actuators. For example, in the case of the ISC (idling speed control) actuator commonly utilized in automobiles for controlling the air intake of an automotive engine, the average current is maintained at a target level by a current control circuit which turns on and off a transistor at a predetermined frequency and controls the average current by adjusting the duty factor (the on/off ratio) thereof.
FIG. 1 shows the circuit organization of a conventional current control circuit for an ISC actuator. A pulse signal from a microprocessor, etc., is input to an input terminal 1 to drive a power transistor 2, which controls the current flowing through the coil 3 of the actuator. A free-wheeling diode 5 is coupled across the coil 3 to pass the current therethrough when the power transistor 2 is turned off. The current is supplied from the battery 21.
The current flowing through the coil 3 rises during the time when the power transistor 2 is turned on by an input pulse applied thereto via the input terminal 1; it decreases while flowing via the diode 5 when the power transistor 2 is turned off. The average current through the coil 3 can be controlled by the duty factor of the input pulse train supplied to the input terminal 1, since the pulse repetition frequency is sufficiently great.
In the case of the above conventional current control circuit for an electromagnetic type actuator, the average current through the coil 3 may vary with time due to the variations of the source voltage, or due to the changes of the coil resistance, etc., which is caused by the heat generated by the current flowing therethrough. Thus, the duty factor of the pulse signal supplied to the current control circuit is usually adjusted by a microcomputer in accordance with the level of the source voltage. However, the variation of the average current caused by the changes of the coil resistance, etc., is not adjusted for by the microcomputer, and hence the operation of the actuator lacks precision, and a precise control of the air intake is impossible.